DEVELOPMENT AND SUCCESSION OF TEETH IN PERAMELES, 447 
Tue Question or NoMENCLATURE OF THE PREMOLARS. 
After careful consideration we have not thought it well to 
adhere to Thomas’s (5) numerical determination of the teeth in 
the premolar region. That is doubtless an attractive theory, 
which aims at enabling us to homologise the premolars of 
modern Marsupials with those of the higher mammals; but 
we are as yet unconvinced that the case has been sufficiently 
made out. 
Admitting the probability that one of the premolar series 
has been lost in the ancestors of modern Marsupials, we 
cannot regard Thomas’s contention that it is p. 2 which has 
disappeared as placed beyond all reasonable doubt. Thomas’s 
case for the homology of the last premolar of modern Mar- 
supials to p. 4 of other mammals is made up of the following 
factors : 
(a) The existence of variations in the way of an occurrence 
of the hypothetically missing p. 2; to which may be added— 
(6) The alleged occurrence in ontogenetic development of 
a possible rudiment of an enamel-germ in the position of 
Thomas’s “ p. 2.” 
(c) The probable phylogenetic relation of Triconodon to 
modern Marsupials. 
The present writers are, however, of the opinion that there 
are too many uncertainties connected with each of these 
factors to warrant our basing any system of nomenclature 
upon a theory so conditioned. 
With regard to the first of these, Bateson (21) has well 
shown how indecisive is the evidence derived from a study of 
tooth-variations in determining individual homologies of teeth. 
In Bateson’s judgment ‘the system elaborated by Thomas 
breaks down ; not because there is any other system which can 
claim to supersede it, but because the phenomena of variation 
are not capable of this kind of treatment,” because “it is not 
possible to apply any scheme based on the conception that 
each tooth has an individual homology which is consistently 
